Four productions of Handel operas — an “Almira,” an “Aci, Galatea e Polifemo” and two takes on “Radamisto” — testify to the popularity of early music. In November, Juilliard Opera combined forces with Juilliard415, the orchestra of the historical performance program, in a spare but effective staging of “Radamisto” in the school’s Peter Jay Sharp Theater, directed by James Darrah and conducted by Julian Wachner, the moving force behind that surge in Trinity Wall Street’s music program. Two singers — Mary Feminear, soprano, and Virginie Verrez, mezzo-soprano — stood out in a generally fine cast of young performers.